1. Field of the Invention
The present invention relates to a nebulizer for spraying a liquid with high efficiency, and particularly to a nebulizer suitable for use in an inductively coupled plasma/mass spectrometry system (ICP-MS), an inductively coupled plasma (ICP) atomic emission spectrometry system and an atomic absorption spectrometry system used for inorganic substance analysis.
2. Description of the Related Art
In analytical apparatuses for inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), etc., aerosol is produced from a solution sample by a nebulizer and introduced into a plasma. Here, substances to be analyzed are brought into atomization, excitation and ionization. Owing to a mass analysis for the resultant ions or a spectrometric analysis for light emitted from excited atoms or ions, the identification and determination of each substance to be analyzed present in the liquid sample are realized. A concentric glass nebulizer is often used as the nebulizer. A description related to ICP-AES is disclosed in, for example, Analytical Chemistry, 54(1982), p.533-p.537. At an end of each spray tube, atmospheric pressure becomes less than or equal to 1 atom. by a spray gas. A difference in pressure between the two ends of the tubes is used so that the liquid sample is sucked into the nebulizer from a container. The flow rate of the gas is 1.0 L/min. and the flow rate of the liquid is about 1.0 mL/min.
A micro concentric nebulizer (MCN) related to ICP-MS has been described in Journal of Analytical Atomic Spectrometry, 11(1996), p.713-p.720. A liquid sample is delivered to a single capillary and sprayed around its end by gas which passes therethrough. The flow rate of the gas is about 1.0 L/min. Since the velocity of the gas is faster than that for the concentric glass nebulizer, the efficiency of its spraying is relatively high. However, the introduced flow rate of a sample solution for realizing high-efficiency spraying is limited. The efficiency of the spraying is reduced when the flow rate thereof is 50 xcexcL/min or more.
There is need to prevent deposition of a metal due to heat generated upon cutting work, polishing, etc. Thus, a description related to a spray-like body supply device intended for cooling has been disclosed in Japanese Patent Application Laid-Open No. Hei 8-99051. If a liquid is produced or formed in spray form, then cooling can be carried out more effectively. The device has capillaries through which the liquid flows, and an injection hole (nozzle) from which a spray gas (air) is discharged. The cooling liquid is divided into a plurality of the capillaries, and the ends of the plurality of capillaries are packed into a bundle. The liquid is sprayed at the ends thereof by an air flow discharged through one injection hole. The nozzle is shaped in tapered form.
Japanese Patent Application Laid-Open No. Hei 7-306193 describes a sonic spray ionization technology. A quartz capillary (whose inner and outer diameters are 0.1 mm and 0.2 mm respectively) in which a liquid is introduced, has an end inserted into an orifice (whose inner diameter is 0.4 mm). A high-pressure nitrogen gas introduced inside an ion source is discharged into the air through the orifice, and the liquid is sprayed by a sonic gas flow formed at this time. Gaseous ions are produced in aerosol produced by the spraying. In the present ionizing method, the production of fine droplets by the sonic gas flow essentially plays an important role. The liquid in the sonic gas flow is torn off by a gas flow fast in velocity to thereby produce droplets. The non-uniformity of the concentrations of positive and negative ions in droplets firstly produced by spraying becomes pronounced as the size of each droplet becomes fine. Further, some of the liquid are separated from the surface of the droplet by a gas flow, whereby charged fine droplets are produced. Such fine droplets are evaporated in a short time so that gaseous ions are produced. While the size of each produced droplet decreases with an increase in the velocity of flow of gas, the droplet size increases as the velocity of flow of gas enters a supersonic region. This is because a shock wave is produced in the case of the supersonic flow, and the production of fine droplets is depressed. Therefore, according to the sonic spray ionizing method, when the gas flow is sonic, the finest droplets are produced and the produced amount of ions reaches the maximum. The present method discloses that when the flow rate of the spray gas is 3 L/min., a sonic gas flow is formed.
A sonic spray nebulizer has been described in Analytical Chemistry, 71(1999), p.427-p.432. The nebulizer is similar in structure to the ion source for sonic spray ionization. The inner diameter of a resin orifice is 0.25 mm and a quartz capillary (whose inner and outer diameters are 0.05 mm and 0.15 mm respectively) is used. Since a sonic gas flow is used in a spray gas, the present nebulizer is capable of producing extremely fine droplets. As a result, the spray efficiency of a liquid is greatly improved as compared with the conventional glass nebulizer. In the sonic spray nebulizer, the flow rate of the gas is fixed to the condition for the generation of the sonic gas flow, and the flow rate of a liquid sample is controlled by a pump. The flow rate of the gas ranges from 1.0 L/min. to 1.4 L/min., and the flow rate of the liquid ranges from 1 xcexcL/min. to 90 xcexcL/min.
On the other hand, a nebulizer using a supersonic gas flow has been described in Japanese Patent Application Laid-Open No. Hei 6-238211 and U.S. Pat. No. 5,513,798. The present nebulizer is characterized in that a supersonic gas flow is helically produced in the neighborhood of a liquid outlet at an end of a capillary by a helical gas path. Further, a cylindrical path is placed on the downstream side from an orifice unit and a shock wave of a supersonic gas flow is repeatedly reflected by the inner surface of the path. Since the shock wave collides with a liquid flow many times in an in-path central portion, droplets are efficiently produced from the liquid cut to pieces. The length (corresponding to the distance between the end of the capillary and the surface of the cylindrical path, which is brought into contact with the air) is as about twice as the diameter of the cylindrical path. The flow rate of gas ranges from 50 L/min. to 60 L/min., and the flow rate of the liquid ranges from 91 mL/min. to 100 mL/min. Since the spray gas helically circles round, the formation of a gas flow concentrically with the capillary as described in the prior art is not carried out. The velocity of flow of the spray gas is divided or resolved into a horizontal direction and a vertical direction with respect to the axis of the capillary. While the velocity of flow of the gas is supersonic, a flow velocity component horizontal to the capillary axis is considered to be less than or equal to the speed of sound. In a droplet producing process, the application of the shock wave to the liquid is important and no emphasis is placed on the tearing off of the liquid by a high-speed gas flow.
Upon vaporization of the liquid, the flow rate of fully-vaporizable water per gas flow rate 1 L/min. is about 20 xcexcL/min. at most if calculated from saturated vapor pressure at 20xc2x0 C. Therefore, if sample solution given at a flow rate of 20 xcexcL/min. or more is introduced into an ideal nebulizer when the flow rate of the gas is about 1L/min., then the efficiency of its spraying should have been reduced in the ideal nebulizer. However, an actual nebulizer shows a tendency to improve analytical sensitivity even if the sample flow rate is 20 xcexcL/min. or more. This is because the spray efficiency of the liquid is considered not to have reached an ideal level.
In the concentric glass nebulizer, the flow rate of the liquid is about 500 xcexcL/min. when the liquid is automatically sucked. Therefore, the full vaporization of liquid cannot be carried out when the flow rate is a gas flow of about 1 xcexcL/min. Since a gas flow path is narrow and long structurally, the gas introduced into the nebulizer suffers a pronounced pressure loss in the neighborhood of a jet or injection port or outlet. As a result, the flow velocity of the spray gas is much slower than the speed of sound and the size of each produced droplet is about 10 xcexcm. Most of droplets produced by spraying are coagulated or condensed, whereby they are released from aerosol so as to return to the liquid. Therefore, the spray efficiency of the liquid become extremely low and reaches 1% to 3%. Further, the nebulizer is capable of suitably setting the flow rate of a sample solution through the use of a pump. However, a problem arises in that when the flow rate of the sample solution to be introduced is 300 xcexcL/min. or less, the spraying becomes unstable and hence the nebulizer cannot be used. Therefore, the nebulizer cannot be coupled or linked to a semi-micro liquid chromatographor (liquid flow rate of about 200 xcexcL/min.). (An elementary analytical apparatus might be used to perform a chemical speciation analysis as well as an elementary analysis. In this case, a sample liquid is separated according to the semi-micro liquid chromatography and the separated liquid is introduced into the nebulizer). Even if the flow rate of the liquid sample is increased in a range from 400 xcexcL/min. to 1000 xcexcL/min., the sensitivity of the analytical apparatus little increases. This shows that the substantial amount of the sample introduced into a plasma does not increase.
The micro concentric nebulizer is different from the concentric glass nebulizer, and reduces the flow rate of the liquid sample and improves the efficiency of its spraying. This is because the flow velocity of the gas is considered to be high as compared with the concentric glass nebulizer. Therefore, the micro concentric nebulizer is characterized in that a liquid sample available by a small quantity can be analyzed. However, the flow rate of a liquid sample, which allows the maintenance of high spray efficiency, is less than or equal to 50 xcexcL/min. When the flow rate thereof is greater than that, the sensitivity of the analytical apparatus little increases. As a result, the micro concentric nebulizer is accompanied by a problem in that when liquid samples identical in concentration are analyzed, the sensitivity of the analytical apparatus is low as compared with the use of the concentric glass nebulizer. A problem arises in that particularly when a chemical speciation analysis which uses a semi-micro liquid chromatograph jointly, is performed, the flow rate of a liquid reaches about 200 xcexcL/min. and the sensitivity of the analytical apparatus is insufficient.
The sonic spray nebulizer has a problem similar to the micro concentric nebulizer. While the present sonic spray nebulizer is a nebulizer capable of introducing a liquid given at a low flow rate with high efficiency, it uses a sonic gas flow for the purpose of liquid spraying. Since a wide gas flow path is provided therein, a pressure loss of gas is very low and the sonic gas flow can easily be formed. Further, since the end of a capillary in which the liquid is introduced, is placed in the center of an orifice used as a gas jet port or outlet, the efficiency of spraying is extremely high. However, the flow rate of a liquid sample, which allows the implementation of high spray efficiency, is about 60 xcexcL/min. or less in the sonic spray nebulizer in a manner similar to the micro concentric nebulizer. When the flow rate is greater than that, the spray efficiency is reduced and the sensitivity of an analytical apparatus does not increase significantly.
As described above, the liquid is sprayed through the use of the high-speed (sonic) gas flow in the concentric glass nebulizer, the micro concentric nebulizer, and the sonic spray nebulizer. These nebulizers are respectively accompanied by a problem in that while spraying is carried out through the single jet or injection port or outlet, the spray efficiency is reduced with an increase in flow rate when the liquid flow rate is greater than or equal to about 60 xcexcL/min. When they are installed in a plasma mass analyzer or a plasma atomic emission spectrometry system, it is necessary to properly use nebulizers such as a glass nebulizer, etc. according to the flow rate of the liquid sample, thus causing inconvenience. Particularly when the chemical speciation analysis is done which uses jointly a semi-micro liquid chromatograph in which the liquid flow rate is about 200 xcexcL/min., a problem arises in that, for example, the spraying becomes unstable, thereby making each nebulizer incapable of use, and the spray efficiency becomes low.
As indicated by the sonic spray ionization technology, the size of each produced droplet depends on the gas flow rate. When the flow rate of the spray gas is sufficiently high, the spray efficiency reaches the maximum in the case of the sonic gas flow owing to the effects of tearing off the liquid by the sonic gas flow, and hence the spray efficiency of the liquid becomes high. Japanese Patent Application Laid-Open No. Hei 9-239298 discloses that when a gas flow rate is 3 L/min., a sonic gas flow is formed. Thus, when no limitation is imposed on the gas flow rate relative to the liquid flow rate, the size of each of droplets produced by spraying reaches the minimum upon the speed of sound, and hence droplets each having a sub-micron size of about 0.7 xcexcm are produced in large quantities. When a gas flow slightly faster than the speed of sound is used, the size of each droplet actually tends to increase on the average, but droplets of sub-micron sizes are produced. However, a limitation is often imposed on the flow rate of a usable spray gas in an actually-used nebulizer. In the plasma atomic emission or mass spectrometry system, for example, the flow rate of the spray gas is required to set to about 1 L/min. A restriction is imposed on the gas flow rate relative to the liquid flow rate in order to increase the spray efficiency of a liquid to the maximum under the condition that the gas flow rate is kept constant. As a result, the size of each droplet reaches the minimum where a supersonic gas flow other than the sonic gas flow is formed. This is because even if a shock wave for restraining or controlling the scale-down of each droplet is formed, the production of fine droplets by a gas-flow tearing-off effect becomes effective if a gas flow faster than the speed of sound is formed. Namely, it is desirable that the velocity of flow of the gas is supersonic rather than sonic or less in order to produce droplets each having a sub-micron size in large quantities at a constant gas flow rate and increase the spray efficiency of the liquid to the maximum.
In the nebulizer described in U.S. Pat. No. 5,513,798 which aims to spray a large quantity of liquid in a large quantity of gas, a supersonic spray gas flow is used. In the present nebulizer, the gas flow is not formed concentrically with the capillary as described in the prior art, and the spray gas helically circles round. Droplets of 2 xcexcm to 10 xcexcm are produced by applying a shock wave to the liquid without using the effects of tearing off the liquid by a high-speed gas flow. Since each droplet is large and micron in size, it is difficult to implement an increase in the sensitivity of each system even if the present nebulizer is used as nebulizers for a spectrometry system and a measuring system. Further, since the helical gas flow is formed, the gas flow path is structurally narrow and complex, and the pressure of gas introduced into the nebulizer reaches significant high pressure. A problem arises in terms of fabrication upon applying such a nebulizer to the case where the gas flow is low. While the gas flow rate is set to 1 xcexcL/min. in the nebulizers for the ICP atomic emission spectrometry system and the ICP-MS in particular, it is extremely difficult to fabricate a nebulizer which copes with such a low gas flow rate. This is because the gas flow path needs high-accuracy micro-fabrication for the purpose of forming the helical gas flow. A problem arises in that since the high-pressure gas is used, a gas supply means as well as the nebulizer also needs to have high pressure resistance.
In the nebulizer described in Japanese Patent Application Laid-Open No. Hei 8-99051, the introduced liquid is divided into the large number of capillaries. The ends of the large number of capillaries are packed into the bundle, and the liquid is sprayed by the gas flow discharged from one jet or injection port or outlet. However, the capillaries lying in the center of the bundle are hard to contact the gas flow, and the spray efficiency becomes relatively low. The structure of the nozzle shaped in tapered form suffers a noticeable pressure loss, and a high-speed gas flow is hard to occur.
In the conventional nebulizers as described above up to now, the sufficient spray efficiency was not always obtained and a limitation was imposed on an applicable liquid flow-rate range from the viewpoint of the production of the droplets of sub-micron sizes in large quantities.
An object of the present invention is to provide nebulizer with high spray efficiency, which is capable of producing droplets of sub-micron sizes in large quantities from a wide range of liquid flow rates under a limited gas flow rate.
In order to solve the above problems, the present invention provides a nebulizer which effectively makes use of the momentum of a gas flow for purposes of liquid spraying by using a supersonic spray gas flow lying in the axial direction of a capillary (flow path). Further, the present invention provides a nebulizer provided with a plurality of spray units.
How to increase an opportunity to allow a spray gas and a liquid to collide with each other in a limited time or space is of extreme importance upon spraying the liquid using a compressed gas. Therefore, a solution and gas are uniformly distributed to each individual spray units to thereby make it possible to increase the probability of contact between the solution and the gas.
When the limited spray gas is distributed to the respective spray units, the flow rate thereof is greatly reduced. In order to improve the effects of making collision between the spray gas and the liquid, a supersonic gas flow having much momentum as compared with a sonic gas flow is used. As a result, fine droplets can be produced with satisfactory efficiency.